Detailed analysis of hourly “pings” to and from a stationary satellite positioned over the central Indian Ocean has allowed authorities to conclude definitively that Malaysia Airlines Flight 370 must have gone down in the southern part of the ocean.

Days after the plane disappeared, analysts at satellite provider Inmarsat used the precise time delay during transmission of those pings to calculate how far the plane was from the satellite, defining two vast circular arcs sweeping across south-central Asia and the Indian Ocean.

Now, an analysis of the shifting frequency of the signals — in the way the sound of a police siren changes as it comes toward you, then speeds away — and comparisons with signals from other planes in the region at the time has allowed the analysts to discount the northern arc and to zero in on a more defined final known position in the southern Indian Ocean, west of Australia.

“They’ve at least been able to narrow down to a fairly localized region” of that southern arc, said Tim Farrar, president of TMF Associates, a Menlo Park, Calif.-based satellite consulting company.

Malaysian Prime Minister Najiv Razak in a news conference Monday said this new analysis confirms beyond reasonable doubt that the plane soon afterward must have run out of fuel and crashed into those waters, and that all 239 people aboard must be dead.

“This is a remote location, far from any possible landing sites,” Najiv said.

Farrar said further analysis of the satellite signals could help narrow the search area further.

It could also help determine whether the plane was traveling for the last part of its journey in a straight line at a constant speed, with no apparent pilot intervention.

If so, that would raise the possibility that the plane was on autopilot before plunging into the ocean, with the pilots — and perhaps the passengers — incapacitated, possibly already dead.

Najiv said the conclusion the jet is lost in the location identified was arrived at “using a type of analysis never before used in an investigation of this sort.”

Tracking via satellite pings

Airliners use a satellite connection while out of radio range over the ocean for voice communication with the pilots and also to carry text messages to the airline operations base via the jet’s data-communications link, the Aircraft Communications Addressing and Reporting System, or ACARS.

ACARS sent its last transmission a half-hour after takeoff. The last radio contact with the plane was 38 minutes into the flight; the jet dropped off radar minutes later.

Even if ACARS was either turned off by someone or ceased functioning because of some system failure, the satellite connection remained.

Just as a cellphone communicates with cell towers so that the network can reach it with an incoming call, the Inmarsat satellite sends an hourly message to the terminal on board the airplane to check that it is still connected.

In response, the terminal sends back a ping to say, yes, it’s still there.

The final ping from Flight 370 was sent at 8:11 a.m. local time, 7½ hours after the jet took off from Kuala Lumpur.

The pings identified the airplane as Flight 370 but contained no direct location data.

Nevertheless, Farrar, who said he spoke with both Inmarsat and the designers of the satellite equipment on the airplane, explained how it’s possible to use the raw ping data to deduce location information.

The time the response ping takes to travel from the plane to the satellite hovering 23,000 miles above the Indian Ocean varies according to the distance the signal must traverse.

The satellite recorded at what point in the predetermined transmission time slot the ping from Flight 370 arrived, and so established how far away the jet was.

It was this data that persuaded the Malaysian authorities, a week after the plane disappeared, to reorient the search toward two
arcs in a large radius around the satellite.

One arc swept north over land into Central Asia, the other south into the remote southern ocean.

Not totally stationary

Subsequent analysis used the fact that the Inmarsat satellite is not perfectly stationary in its orbit.

Although it is fixed on the 64-degree east line of longitude, the satellite moved slightly from north to south of the equator during the jet’s flight, which means it was moving away from jets in the Northern hemisphere and moving toward jets in the Southern hemisphere.

The Doppler effect — the same phenomenon that changes the pitch of a siren as an ambulance moves toward, past and then away from a listener — enabled analysts to determine whether the plane is moving toward or away from the satellite as it pinged.

The frequency of the Flight 370 signal told analysts it was in the southern arc, not the northern.

By comparing the signals from other planes in the sky at the time that were pinging the same satellite and whose position was known, they were able to further narrow the jet’s possible location, Farrar said.

The search remains daunting. The area where it will now be concentrated, toward the southern tip of the original search arc, is still many tens of thousands of square miles.